There are two principal ways to pay for mobile telephone service: the prepaid or “pay-as-you-go” model where voice, text, and data communication services are paid for before use and added to a phone unit via an Internet account or in retail stores, or the postpaid contract model where bills are presented for payment after the service has been consumed. Users may purchase a basic prepaid or postpaid package and then add on services and functionality to create a subscription customized to the users' needs. With prepaid plans, purchased credit is used to pay for mobile phone services at the point the service is accessed or consumed. If there is no available credit then access to the requested service is denied by the service provider. Users may add to or “top up” their credit at any time using a variety of payment mechanisms. Credit purchased for a prepaid mobile phone may have a time limit, for example ninety days from the date the last credit was added. Postpaid contract users are billed after the fact according to their use of mobile services at the end of each billing period. The customer's contract typically specifies a limit or allowance of minutes, text messages etc., and the customer will be billed at a flat rate for any usage equal to or less than that allowance. Any usage above that limit incurs extra charges.